Plants and microbes synthesize a large number of natural substances, in particular secondary metabolites, with diverse and generally unclear function. In contrast to the primary metabolites (e.g. amino acids, sugars, fatty acids), which are involved in fundamental functions like metabolism, growth, maintenance and survival, secondary metabolites are not required for fundamental functions. Many secondary metabolites from plants are known to act as repellants or natural pesticides in defense against herbivore animals or as sexual attractants to pollinating insects (Grisebach, 1988, In: European Conference on Biotechnology, Scientific, technical and industrial challenges, Verona, Italy, 7-8 Nov. 1988, pages 23-27), whereas fungal secondary metabolites often act as phytotoxins (Osbourn, 2001, Proceedings of the National Academy of the USA 98: 14187-14188).
Various secondary metabolites from more than 100 different plant species have been shown to exert antimicrobial activity (Cowan, 1999, Clinical Microbiology Reviews 12: 564-582) and a large number of secondary metabolites from common food plants are not only responsible for the taste and color but are also believed to have health promoting activities (Eastwood, 2001, Quarterly Journal of Medicine 94: 45-48; Drewnowski & Gomez-Carneros, 2000, American Journal of Clinical Nutrition 72: 1424-1435). Accordingly, these natural substances are economically important in such different fields as drugs, food additives, fragrances, pigments, and pesticides.
Secondary metabolites often accumulate in small quantities and sometimes only in specialized cells. Hence their extraction can be difficult and inefficient. In spite of the progress in organic chemical synthesis, a large number of these metabolites have such complex structures that they are virtually impossible to synthesize at economic levels. Moreover, the natural product is generally more acceptable to consumers than an artificially produced one. Consequently, industrial application of these substances and their functional analogues often relies on natural extraction from plants.
Secondary metabolites may generally be structurally qualified as low molecular weight organic compounds.
Industrial production of secondary metabolites or other natural and non natural low molecular weight organic compounds can be facilitated by a biotechnological approach. By transformation of genes involved in the biosynthesis of a desired natural product, plants or microbes can e.g. be manipulated to produce a compound not previously present in the plant or organism.
Glycosyltransferase may be defined as an enzyme which transfers residues of sugars (galactose, xylose, rhamnose, glucose, arabinose, glucuronic acid, etc) to acceptor molecules. Acceptor molecules may be other sugars, proteins, lipids and other organic substrates. The acceptor molecule may be termed an aglycon (aglucone if sugar is glucose). An aglycon may be defined as the non-carbohydrate part of a glycoside. A glycoside may be defined as an organic molecule with a glycosyl group (organic chemical group derived from a sugar or polysaccharide molecule) connected to it by way of e.g. an intervening oxygen, nitrogen or sulphur atom.
These glycosylated molecules take part in diverse metabolic pathways and processes. The transfer of a glycosyl moiety can alter the acceptor's bioactivity, solubility, stability, taste, scent and transport properties e.g. within a plant or microbial cell and throughout the plant.
The art describes a number of glycosyltransferases that can glycosylate compounds such as secondary metabolites from e.g. plants and fungi (Paquette, S. et al, Phytochemistry 62 (2003) 399-413).
WO01/07631, WO01/40491 and (Arend, J et al., Biotech. & Bioeng (2001) 78:126-131) describe that at least some of these glycosyltransferases are capable of glycosylating a number of different structurally related secondary metabolites and other structurally related low molecular weight organic compounds.
Accordingly, the skilled person has at his disposal a number of different glycosyltransferases capable of glycosylating numerous different secondary metabolites and other structurally related low molecular weight organic compounds.
Tattersall, D B et al, Science (2001) 293:1826-8 describes that the entire pathway for synthesis of the tyrosine-derived cyanogenic glucoside dhurrin [a seconday metabolite] has been transferred from the plant Sorghum bicolor to the plant Arabidopsis thaliana. The entire pathway for synthesis included two genes involved in the biosynthesis pathway (CYP79A1 and CYP71E1) and a glucosyltransferase (sbHMNGT) capable of glucosylating the last intermediate (p-hydroxymandelonitrile) to get the glucoside dhurrin (see FIG. 1 herein). It was demonstrated that the transgenic Arabidopsis thaliana plant was capable of producing 4 mg of dhurrin per gram of fresh weight.
Arend, J et al., Biotech. & Bioeng (2001) 78:126-131 and WO01/07631 describes cloning of a glucosyltransferase from the plant Rauvolfia serpentina. The cloned glucosyltransferase was inserted into E. coli bacteria. When the aglucones hydroquinone, vanillin and p-hydroxyacetophenone were added to the medium of cultivated cells of the engineered E. coli, the corresponding glucosides, arbutin, vanillin-D-glucoside and picein were synthesized. They also were released from the cells into the surrounding medium.
Moehs, C P et al, Plant Journal (1997) 11:227-236 describes that a cDNA encoding a solanidine glucosyltransferase (SGT) was isolated from potato. The cDNA was selected from a yeast expression library using a positive selection based on the higher toxicity of steroidal alkaloid aglycons relatively to their corresponding glycosylated forms. The activity of the cloned SGT was tested in an in vitro assay based on isolated recombinant produced SGT.
U.S. Pat. No. 6,372,461 describes a method for making the secondary metabolite vanillin by use of an E. coli cell where there has been introduced genes involving in the biosynthesis pathway starting from glucose and leading to vanillic acid. The recombinant E. coli can produce vanillic acid when cultured in a medium comprising glucose. The produced vanillic acid is recovered from the fermentation broth and reduced to vanillin with aryl-aldehyde dehydrogenase.